1. Field of the Invention
The invention relates to interferometers and, more particularly, relates to a geometrically-desensitized interferometer (GDI) instrument for surface profiling. Even more particularly, the invention relates to a full-field GDI instrument combining diffractive optics and conventional optics to perform beam splitting and beam recombining operations. The invention additionally relates to a method of surface profiling using a full-field GDI instrument.
2. Discussion of the Related Art
Optical metrology of surface profiles can generally be divided into two regimes, namely interferometric and geometric. Geometric techniques include triangulation and moire fringe analysis, which involves the projection and imaging of a periodic structure such as a ronchi ruling. Geometric techniques are relatively insensitive to surface roughness and deformations, but are of relatively low resolution--so low, in fact, that they are unsuitable for many applications in which surface profiles must be measured with high precision.
Interferometry, on the other hand, relies on the wave nature of light to ascertain with high precision the surface profile of a test object. A typical traditional interferometer includes a light generator that generates a beam of light, a spatial filter-beam diverger that diverts the light beam into a diverging spherical wavefront, a beam splitter that diverts part of the diverging spherical wavefront from the filtered beam, and a collimating lens that collimates the wavefront to produce a piano wavefront of coherent light. This wavefront of coherent light is then reflected off test and reference surfaces to produce first and second reflected wavefronts which combine with one another while interfering both constructively and destructively to produce an interference fringe pattern. An imaging device such as a solid state camera receives the recombined wavefronts and acquires images of the interference fringe pattern. The interference fringe pattern then is analyzed to obtain information about the surface profile of the test object.
Fringe pattern analysis for surface profilometery often is performed by the well-known technique of phase shifting interferometry (PSI). In PSI, the height difference between locations on a surface imaged by first and second pixels on the imaging device is determined by first determining a phase difference between light received at the first and second pixels and by then using the phase difference to calculate a height difference. A primary advantage of PSI is that it is highly precise. The vertical height precision for PSI is a fraction (e.g., 1/100) of the optical wavelength of the light source used to conduct the measurement. A second advantage of PSI is that it has good vibration immunity characteristics because phase data is acquired for all pixels simultaneously and because the data acquisition time is relatively short.
Generally speaking, however, conventional PSI approaches can only profile smooth surfaces having relatively small height variations or "surface departures" between adjacent measurement sites. This constraint results from the fact that PSI has a phase ambiguity constraint. Specifically, the maximum physical departure between adjacent measurement sites on the profiled surface must be less than 1/4 of the source wavelength. Stated another way, the maximum phase difference between the reference and test light beams must have an absolute value which is less than .pi.. This constraint, sometimes known as "two .pi. ambiguity", arises because the arctangent function, which is used to convert phase to distance, is only unique within the range of .+-..pi.. Thus, although the use of phase measurements advantageously allows very high precision to be obtained, it disadvantageously limits the maximum surface departure between adjacent measurement sites to one quarter of the source's optical wavelength. A further difficulty with PSI arises when the surface slope is so large that it becomes difficult to resolve or distinguish the interference fringes because the fringe density is too high. Therefore, while PSI interferometetry is much more precise than geometric optical profilometery, it historically has been considered to be ill-suited for use with rough objects or objects having marked surface deformations. Interferometers using PSI analysis therefore historically have not been considered appropriate for some surface profilometery applications.
One interferometric technique that lacks the quarter-wavelength constraint of PSI is the so-called scanning white light interferometry or SWLI. In SWLI, a white light illumination source or, more generally, one which is of a broad-band as opposed to being of a narrow-band, generates an interference pattern which contains regions of high contrast for each location on the test surface as a function of scan position. The scan position of high contrast for a given pixel indicates the height of the corresponding location on the test surface. Therefore, by comparing the temporal characteristics of these regions of high contrast with one another, a difference in height between two locations on the profiled surface can be determined. Unlike PSI, SWLI does not calculate height differences based on phase differences, and the PSI phase constraint therefore does not apply to SWLI. The maximum physical departure between adjacent measurement sites on a profiled surface therefore may be much larger with SWLI than with PSI.
However, SWLI has disadvantages of its own that hinders its use in industrial applications. For instance, the field of view is generally no larger than can be accommodated by standard microscope objectives. To function correctly, the imaging device of the instrument must have high resolution when compared to the corresponding interference fringe density. When the field of view of the typical SWLI instrument is increased, the fringe density can easily become difficult to resolve even with very high resolution imaging devices. This problem is especially evident during the profiling of rough surfaces. Moreover, slope tolerance for specular surfaces decreases linearly with the field size, and the speckle effects required for rough-surface measurements are only resolvable if the numerical aperture (NA) of the objective decreases linearly as the field increases. The need to resolve the speckle pattern from rough surfaces is the most discouraging, since the amount of collected light decreases with the square of the NA. The light loss means that larger surfaces require a more powerful illuminator. Worse, the fringe contrast is now a highly variable parameter, and the quality of the measurement depends critically on the balance between the reference and object beam intensities.
Another disadvantage of typical SWLI techniques is that data acquisition is very slow. The slow speed is a consequence of the large amount of data which must be acquired given the rapidly varying interference effect as a function of scan position. Accurate measurements require that these variations be recorded in detail, usually at the rate of one measurement per pixel per 75 nm of scan motion. The slow speed creates additional problems such as a high sensitivity to thermal distortions and mechanical strain during measurement.
Still another disadvantage of typical SWLI is its high sensitivity to vibration, which is due in part to the slow data acquisition speed, and in part to the extremely high sensitivity of the interference fringe pattern, which is easily corrupted by very small amounts of vibration. An instrument configured for SWLI analysis generally requires massive mounting fixtures and expensive vibration isolation. Even with these precautions, such instruments are still restricted to relatively vibration-free environments as compared to normal production environments.
Recent years have seen an increased demand for the high speed, high precision metrology of the surface profiles of manufactured parts having large surface departures, i.e., having rough surfaces or surfaces with pronounced surface deformations. A corresponding demand has arisen for the acquisition of data during production as opposed to in the laboratory. For instance, precision products such as hard disks for computer disk drives need to be profiled with high precision, at high speeds, and under conditions in which the test object may be subjected to substantial vibrations during manufacturing processes. Neither traditional PSI techniques nor traditional SWLI techniques are suitable for these purposes. A need therefore has developed for a "desensitized" interferometer that is relatively insensitive to surface roughness and surface deformations, that performs surface metrology with high accuracy and at high speeds, and that is relatively insensitive to vibrations and therefore is well-suited to production-line use.
This need has been met to a large extent by the development of the geometrically-desensitized interferometer (GDI) instrument. A GDI instrument is characterized by the replacement of the beam splitter of the traditional instrument with an optical assembly located between the collimating lens and the test object. The optical assembly, divides the collimated source light into two beams which propagate in two different directions and impinge on the profiled surface at the same location but at different incident angles. The beams reflect from the profiled surface and pass back through the optical assembly in different directions, after which they are recombined. Constructive and destructive interference of the reflected and recombined beams form an interference fringe pattern having an equivalent wavelength .LAMBDA. that may be orders of magnitude larger than the source wavelength. As a result, the GDI instrument is much less sensitive to height variations and surface deformations than are traditional interferometers using PSI analysis techniques.
Some forms of GDI instruments also are achromatic. That is, the fringe spacing in an interference fringe pattern produced by a GDI instrument is independent of the source wavelength. As a result, and unlike with SWLI interferometers, there is no coherence envelope associated with the source bandwidth. Many disadvantages associated with SWLI such as a limited field of view, a slow acquisition speed, and a high sensitivity to vibration therefore are avoided. The sensitivity of GDI instruments is intermediate conventional interferometry and moire fringe analysis, and is comparable to that obtained with grazing-incidence interferometry. GDI instruments therefore can be used in manufacturing applications and other applications that are unsuitable for traditional interferometry.
The two best-known types of GDI instruments are grating-based and conventional optics-based, respectively. The characteristics and limitations of each type of instrument will now be briefly described.
A grating-based GDI instrument is characterized by the use of at least one (and usually two) diffractive gratings that perform all of the beam splitting and beam recombining operations of the instrument. An exemplary grating-based GDI instrument is disclosed in U.S. Pat. No. 5,526,116 to de Groot (the de Groot '116 patent). Specifically, FIG. 2 of the de Groot '116 patent illustrates a diffractive optical assembly that includes first and second parallel linear phase gratings spaced from one another in the Z direction of the instrument. The second grating is not strictly required but produces the advantage of permitting the working distance between the exit surface of the grating assembly and the profiled surface of the test object to be increased from zero to a somewhat larger distance--typically about 2 inches. Both gratings are involved in both the splitting of an inbound beam and in the recombining of reflected beams from the object surface. Specifically, the first grating diffracts an inbound collimated beam from a light source into two first-order beams "A" and "B". The beams A and B are then redirected by the second grating so that they impinge on the profiled surface of the object at the same location but at different incident angles .alpha. and .beta.. Reflected beams A' and B' propagate outwardly from the profiled surface at corresponding angles .alpha.' and .beta.' and travel back through the second and first gratings sequentially so as to recombine with constructive and destructive interference. The recombined interfering beams or wavefronts are then imaged by an imaging device to display an interference pattern representative of the profile of the imaged surface. Typically, the interference pattern provides for each point on the imaged object surface an interference phase that is substantially linearly proportional to the local surface height.
The typical grating-based GDI instrument exhibits all of the above-described advantages of GDI instruments. Moreover, because it only requires two optical devices to split and recombine the inbound and outbound beams, it is relatively compact, relatively easy to align and maintain its alignment, and has a relatively small sensitivity to air turbulence. Moreover, it constitutes a true full-field GDI instrument. A "full-field" instrument is one in which the optical path difference between the two inbound beams A and B (or between the two outbound beams A' and B') is substantially independent of field position on a perfectly smooth sample surface the orientation of which relative to the instrument is adjusted to produce null fringes when it is imaged. The full-field capability of grating-based instruments arises from the fact that the gratings permit a change of direction of the beam without actually tilting the gratings relative to one another. The basic geometry of a full-field instrument is substantially unchanged with either side-to-side or fore-to-aft object tilt and, accordingly, provides a substantially linear and uniform response to surface topography over the entire imaged area of the object surface. In contrast, an instrument that is not capable of full-field imaging provides a substantially linear and uniform response only at a single point or along a single line of the imaged object surface.
Grating-based GDI instruments have some limitations arising from the fact that all beam splitting and recombining operations are performed via the same phase grating(s). For instance, they have a relatively small working distance (defined as the distance between the second grating and the profiled surface) that varies generally directly with the distance between the first and second gratings. For example, a grating-based GDI instrument employing currently-available phase gratings generally has a maximum working distance of about 2". This working distance is sufficiently large for most laboratory applications. However, it is insufficiently large for some industrial applications in which the test object is located in an environment that is hostile to the instrument and/or is separated from the instrument, e.g., by a transparent wall. These applications include profiling an object in an oven or profiling an object in a processing chamber.
Another limitation of the typical grating-based GDI instrument is that it requires the use of relatively expensive and difficult-to-obtain phase gratings. Phase gratings or their functional equivalents are required because these same gratings both 1) split inbound beams to form the converging impinging beams and 2) recombine reflected outbound beams to form the outbound interference beam. Stated another way, the gratings must be capable of accommodating both positive diffraction orders and negative diffraction orders with sufficient transmission efficiency to produce acceptable interference fringes using commercially-available light sources and imaging devices. Phase gratings meeting these requirements are relatively difficult to obtain and cost about twice as much as comparable reflective gratings designed for light impingement on one surface only. Moreover, the basic geometry of the typical grating-based GDI instrument precludes blazing the phase grating to enhance its transmission efficiency.
Conventional optics-based GDI instruments rely on beam splitters, lenses, mirrors, and/or other conventional optical elements to split and recombine beams. GDI instruments of this type are disclosed, e.g., in U.S. Pat. No. 3,958,884 to Smith (the Smith patent) and U.S. Pat. No. 4,714,348 to Makosch (the Makosch patent). For instance, the instrument illustrated in FIG. 6 of the Makosch patent employs a beam splitter and a plurality of mirrors to generate three collimated beams that are brought to interference in a symmetrical light field. When an object surface is brought into this light field, reflected beams are transmitted back through the system of splitters and mirrors and are recombined to form an interference pattern representative of the profile of the imaged surface. Conventional optics-based GDI instruments have the advantage of not requiring phase gratings and hence tend to exhibit a larger working distance than grating-based instruments. For small objects, they also may be less expensive to manufacture than grating-based GDI instruments to the extent that they employ more readily available optics than the phase gratings employed by grating-based GDI instruments.
However, conventional optics-based GDI instruments have at least one very serious limitation that severely limits their practical range of applications. That is, unlike grating-based instruments, conventional optics-based GDI instruments do not have full-field capability. This limitation arises from the fact that the field-independent path difference condition of a full-field instrument is not fulfilled by the mirrors or other conventional optical elements typically used in conventional optics-based GDI instruments. Conventional optical elements cannot be positioned in parallel with one another and still produce beams that overlap on the object surface at two different angles of incidence. Hence, in the Makosch patent, the inbound beams exhibit a first OPD at one field position on the object surface as illustrated in FIG. 6 of the Makosch patent and exhibit a different OPD at a different field position, even if the object surface is perfectly flat. Depending on the manner in which interference data is acquired and interpreted, the actual field of view of the typical conventional optics-based GDI instrument may therefore constitute no more than a point on the object surface relative to the instrument or, at best, a line extending along the object surface. Therefore, conventional optics-based instruments generally are limited to single-point profiling or, at best, linear profiling. This extremely limited field of view is unacceptable for most applications.
A need therefore has arisen to provide a GDI instrument that has full-field imaging capability but that lacks the limitations of grating-based GDI instruments.